Although impressive at an airshow, this is not actually "fuel dumping", which is a procedure done purely to reduce weight, and the fuel is NOT ignited as the pilot usually has more on his mind if a jet is trying to reduce weight in a hurry ! This is a deliberate design feature from some years back when the vast majority of air-to-air missiles were of the "heat seeking" (IR) variety... the idea being you squirt out a small burst of fuel, hit the afterburner to a) ignite it, b) accelerate away very quickly, so leaving a small "lump" of superheated air, with a VERY high IR signature, behind you whilst you move away..... the heat seeking missile heads to the lump of superheated air, not you, and you escape its intent. This was seen as a lighter, more efficient protection system than kitting out an aircraft with the ability of firing decoy flares, which are limited in number and quite heavy.

Modern missiles are rarely heat seeking but usually radar guided, so this is "old tech" now .... but still has a "Wow" factor at the airshow, when the flame is kept alight far longer than required !!